linux-kernel - Re: [PATCH v2 1/4] pwm: kona: Remove setting default smooth type and polarity for all channels

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Message-ID: <5479097B.6020108@broadcom.com>
Date:	Fri, 28 Nov 2014 15:47:07 -0800
From:	Arun Ramamurthy <arun.ramamurthy@...adcom.com>
To:	Tim Kryger <tim.kryger@...il.com>,
	Scott Branden <sbranden@...adcom.com>
CC:	Thierry Reding <thierry.reding@...il.com>,
	Ray Jui <rjui@...adcom.com>,
	Arun Ramamurthy <arunrama@...adcom.com>,
	<bcm-kernel-feedback-list@...adcom.com>,
	<linux-pwm@...r.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] pwm: kona: Remove setting default smooth type
 and polarity for all channels



On 14-11-25 09:51 PM, Tim Kryger wrote:
> On Tue, Nov 25, 2014 at 11:40 AM, Scott Branden <sbranden@...adcom.com> wrote:
>> From: Arun Ramamurthy <arunrama@...adcom.com>
>>
>> The probe routine unnecessarily sets the smooth type and polarity for
>> all channels. This causes the channel for the speaker to click at the same
>> time the backlight turns on. The smooth type and polarity should be set individually
>> for each channel as required and no defaults need to be set.
>
> I am guessing you are talking about a PWM controlled beeper/buzzer.
>
This change is more so to remove setting smooth type and polarity for 
all channels during probe and to leave them as their default values. 
Infact, setting the PWM_CONTROL_TYPE_SHIT is also redundant cause the 
default value is already 1 for all channels. We can remove that loop 
entirely and this will be done in the next patch set. The smooth type 
and polarity are only changed when the particular pwm channel is enabled 
or polarity is changed.

> Can you mention what board you are observing this issue on?
>
> Also please explain why setting these bits result in an audible click.
>
We observe this on the bcm958300K board where one of the
PWM channels is connected to the buzzer and changing the
smooth type and polarity from its default values causes a click

>>
>> Signed-off-by: Arun Ramamurthy <arunrama@...adcom.com>
>> Reviewed-by: Ray Jui <rjui@...adcom.com>
>> Signed-off-by: Scott Branden <sbranden@...adcom.com>
>> ---
>>   drivers/pwm/pwm-bcm-kona.c | 7 ++-----
>>   1 file changed, 2 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/pwm/pwm-bcm-kona.c b/drivers/pwm/pwm-bcm-kona.c
>> index 02bc048..29eef9e 100644
>> --- a/drivers/pwm/pwm-bcm-kona.c
>> +++ b/drivers/pwm/pwm-bcm-kona.c
>> @@ -266,12 +266,9 @@ static int kona_pwmc_probe(struct platform_device *pdev)
>>                  return ret;
>>          }
>>
>> -       /* Set smooth mode, push/pull, and normal polarity for all channels */
>> -       for (chan = 0; chan < kp->chip.npwm; chan++) {
>> -               value |= (1 << PWM_CONTROL_SMOOTH_SHIFT(chan));
>> +       /* Set push/pull for all channels */
>> +       for (chan = 0; chan < kp->chip.npwm; chan++)
>>                  value |= (1 << PWM_CONTROL_TYPE_SHIFT(chan));
>> -               value |= (1 << PWM_CONTROL_POLARITY_SHIFT(chan));
>> -       }
>>
>>          writel(value, kp->base + PWM_CONTROL_OFFSET);
>
> While the smooth bit need not be set here, it is important that the
> polarity bit be set.
>
The default value for polarity is 0 which is normal polarity, so setting 
it to 1 here in the probe function without a sysfs call is
when the software will report the polarity as normal when it is actually 
inversed.
> Otherwise software will report the polarity as normal when it it is
> actually inversed.
>
> Consider the case where a userspace process is controlling the PWM via sysfs.
>
I agree with you about the sysfs case Tim, but since this is the probe 
function and not a sysfs callback, should we not leave it as the default 
value?
--
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